[GTZ]

Hello to all.
Some days ago,I picked up my car for a 250 km trip.50 km away from home low battery indication appeared in display.I checked battery voltage (via OBD bluetooth device) and it was 11,7 volts.Immediately I turned back home of course taking care to avoid electric consumption.The car unfortunately stopped 300 m away from home because of low battery voltage,10,0 volts.
It looks that the altenator was failed.
I connected a trickle charger (CTEK) for 10 minutes and tried to start up the engine because the car was stayed in the middle of the road.Engine was started (with difficulty) and suddenly altenator was present!!!!!!!!!
13,8 volts and everything was OK.All systems were ok,all warnings went away.
I have to say that my battery is a brand new (aged 4 months) VARTA AGM 95AH,registered to DME and the car is always connected to the trickle charger.This means that this morning before driving away from home,the car was connected to the charger and the battery was fully charged.
The point is,why did the altenator failed to start during this trip?
Did it have anything to do with the trickle charger?
If battery is full because of a trickle charger,altenator engages or not?
I have also to say that as a BMW enthusiast I also own BMW Rheingold ISTA+ and always check for errors.10 days before this incident,I mentioned the 210901 code "BSD,message;altenator:missing".I immediately checked voltage with voltage meter to the pins under the hood and 13,8 to 14,4 was measured so I ignored the 210901 code although this code was permaned and not able to be cleared by ISTA+ Rheingold.
Finnaly I brought the car to the specific BMW service of my city (after the incident).Technician said that he never found 210901 code,of course he found codes that have to do with low voltage,as car was stopped in the middle of the road out of electricity.He said that he cleared all codes,he made all the proper tests icluding IΒS test and he found nothing!!!!! So he suggested to drive the car around the city and watch the behaviour because car is OK for him.He also suggested me to stop plugging EBAY Bluetooth OBD adapters as it can be the cause of the problem.These adapters could support all the OBD communication protocols but sometimes could give faulty commands to DME and DME could react faulty too...
The truth is that during this incident,this adapter was plugged in OBD for monitoring reasons (because engine is tuned...)
So,following his suggestion,I made 200+ kms around the city and nothing happen.I entered the secret menu of the instrument cluster and watched system voltage.Values varied from 14,4 volts on braking,to 11.9 volts at coasting.I'm not sure if I can still use the car,so I plugged ISTA+ again to search for codes.Unfortunately 210901 is still present and this means that for sure the BMW technicial didn't pay the proper attention to this.I can not trust anymore his conclusions...
What should I do?
Obviously the altenator is faulty but it still operates...
Should I replace tha altenator?
Thank you in advance!